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The stratigraphy of the Frontier Sandstone Member of the Mancos Shale (Upper Cretaceous) was studied on the south flank of the eastern Uinta Mountains (northwest Colorado and northeast Utah). The Frontier Sandstone Member (about 275 ft thick) is divided into 5 depositional units, A through E, which are identified on outcrop by their stratigraphic position, lithology, and inorganic and organic structures. These depositional units are distinct local stages of a full cycle of deltaic sedimentation in the study area.
Units A and B represent sedimentation under an encroaching shoreline from the west with a gradual vertical grain-size increase from black shale and siltstone to very fine-grained, fossiliferous sandstone. Unit C represents a further regression of the shoreline with the development of a littoral zone in and near the study area. Unit D, representing the height of the constructional
phase of the delta, is a complex of various lithologies which is divided into 4 major facies: coastal swamp, fluvial estuary, barrier island, and neritic marine. The fluvial estuary is open landward to a major distributary channel and exhibits high-energy, fluviatile bed forms. The barrier island sandstone body shows a N65°E orientation and separates 2 major facies--the coastal swamp/fluvial estuary facies on the northwest and the neritic marine on the southeast. Ophiomorpha burrows, indicators of littoral and shallow neritic environments, are common throughout the barrier-island sandstone. Unit E represents the destructional phase of the delta with initial deposition of marine black shale capping the complex of deltaic sediments of unit D. A grainsize increase upward through uni E, to fine-grained sandstone, culminates in the development of linear, offshore sandstone bars in the upper part of the interval. The sandstones have been bioturbated thoroughly by detritus feeders. Paleorelief, caused by the underlying stratigraphy of unit D, influenced the distribution of the unit E sandstone. Paleotopographic depressions found over the previous positions of both the subshore-face facies of the barrier-island sandstone and the coastal swamp facies, received the thickest offshore sandstone bar development, whereas paleotopographic ridges, specifically above the barrier-island sandstone, received the thinnest offshore bar development. No significant paleorelief was present at the end of the deposition of unit E and the beginning of the deposition of the upper shale memb r of the Mancos Shale.
The zircon:tourmaline ratio, determined by heavy-mineral analysis of the sandstones, can be used to differentiate fluvial and marine sandstones. Because zircon (Sp. G. 4.7) and tourmaline (Sp. G. 3.1-3.3) have widely different specific gravities, their relative abundances can be used as sensitive indicators of depositional environments. This sorting is related to the particular energy regime which existed at the time of deposition. Ratios greater than 1:1 are representative of high-energy fluvial sandstones, and ratios less than 1:1 are representative of low-energy marine sandstones. The marine environment can be subdivided further between shallow neritic (barrier islands, offshore bar, and shoreface environments) and deeper neritic with the shallow neritic having abundant heavy miner ls and the deeper neritic having rare heavy minerals.
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